DDOS if port is closed.

I will try to explain with my english skills. I have game server hosted in ovh. My server was lagging and disconnecting from the server cuz of Ddos attack. I try to protect the ip with proxies from few good providers but nothing helpa. Right now i create automatic system to take real ip of the players and put them automatic on my firewall. Mean Port open for specific ip adresses. The port is closed for others. Only players which start my .exe can login to the server.

Btw i hear that they can ddos if the port is closed With UDP Attacks?

Sorry for my english.